新元古代海水化学演化及其对白云石沉淀的控制作用^*

Neoproterozoic seawater chemical evolution and its controls on dolomite precipitation

摘要

新元古代是地球表层系统由低氧、微生物主导海洋向分层氧化海洋转型的关键时期,海水化学结构与碳酸盐沉积体系发生了显著重组。海相白云石胶结物因其同生—准同生的特征,被广泛用于重建古海水化学条件,但其形成机制及控制因素仍存在争议。以全球典型新元古代碳酸盐岩地层为研究对象,综合岩石学及碳、氧、锶、钼、铀和镁同位素等多重地球化学证据,系统梳理了海相白云石胶结物的岩石学特征及其环境意义。地层对比表明,不同地区白云石胶结物在形态组合和同位素组成上虽存在区域差异,但其沉积时序与成因模式具有良好的可比性,指示白云石发育主要受全球性海水化学背景控制。多同位素记录显示,新元古代海水化学并非长期稳定,而是在构造重组、冰期—间冰期振荡及大陆风化通量变化的共同作用下持续演化。白云石的广泛发育反映的并非单一、长期存在的“白云石海”,而是特定海水化学条件在不同构造-气候下的阶段性出现。研究认为,在相对偏高的 Mg/Ca 值背景下,低硫酸盐浓度、高碱度及适宜的氧化还原条件协同作用,可开启白云石沉淀“窗口”。新元古代构造-气候过程通过调控海水化学端元输入与物质循环方式,使该沉淀窗口在全球多次出现,从而解释了该时期白云石的异常富集。本研究从多尺度、多同位素角度系统厘清了新元古代海相白云石形成的关键控制机制,为重建古海洋化学演化及理解深时地球表层系统协同演变提供了重要约束。

Abstract

The Neoproterozoic represents a critical interval in the evolution of the Earth surface system,during which oceans transitioned from low-oxygen,microbially dominated conditions to a stratified and progressively oxygenated state,accompanied by profound reorganization of seawater chemistry and carbonate depositional systems. Marine dolomite cements,owing to their synsedimentary to early diagenetic modes of formation,have been widely used to reconstruct Neoproterozoic seawater chemistry;however,their formation mechanisms and principal controlling factors remain debated. This study synthesizes petrographic observations together with multiple geochemical proxies,including C,O,Sr,Mo,U,and Mg isotopes,from globally representative Neoproterozoic carbonate successions,to systematically evaluate the petrographic characteristics and paleoenvironmental significance of marine dolomite cements. The stratigraphic comparisons indicate that,despite regional variations in cement morphologies and isotopic compositions,the timing of dolomite precipitation and associated genetic patterns are broadly comparable,suggesting a dominant control by a common global seawater chemical background. Multi-isotopic records further demonstrate that Neoproterozoic seawater chemistry did not remain in a long-term steady state,but evolved dynamically in response to tectonic reorganization,glacial-interglacial oscillations,and variations in continental weathering fluxes. The widespread occurrence of dolomite therefore does not reflect a single,persistently existing “dolomite sea,”but instead records the episodic emergence of favorable chemical conditions during different tectonic-climatic stages. Dolomite precipitation windows are inferred to have been opened under a relatively elevated Mg/Ca background,when low sulfate concentrations,high alkalinity,and appropriate redox conditions acted in concert. Neoproterozoic tectonic-climatic processes modulated seawater end-member inputs and material cycling,allowing such precipitation windows to recur at the global scale and thereby accounting for the anomalous abundance of marine dolomite during this interval. This study systematically clarified the key controlling mechanisms of Neoproterozoic marine dolomite formation from a multi-scale and multi-isotope perspective,providing important constraints for reconstructing the chemical evolution of ancient oceans and understanding the co-evolution of the Earth’s surface system in deep time.

Graphical abstract

关键词

新元古代 / 海水化学演化 / 海相白云石胶结物 / 白云石沉淀 / 同位素地球化学

Key words

Neoproterozoic / seawater chemical evolution / marine dolomite cements / dolomite precipitation / isotope geochemistry

0 引言

新元古代(~1000—541 Ma)是地球表层系统发生关键转折的时期,其间海洋化学结构和氧化还原状态均经历了深刻重组(Spence et al., 2016;Li et al., 2026)。这一时期不仅记录了由低氧、微生物主导海洋逐步向氧化海洋的转变,也为寒武纪生态系统的快速演化奠定了环境基础(Wood,2011;崔一鑫等,2025)。构造上,新元古代经历了罗迪尼亚超大陆逐步裂解以及随后的冈瓦纳大陆初始聚合(图1),伴随显著的气候振荡与全球碳循环异常,使海洋-大气-岩石圈体系长期处于非稳态演化之中(Li et al., 2013;Nance,2022)。

~850—720 Ma,罗迪尼亚超大陆裂解加速,新洋盆相继开启,洋中脊活动增强,全球海底扩张与热液交换达到高值阶段(Li et al., 2013)。随后发生的Sturtian和Marinoan冰期显著改变了地表物质循环方式,在阶段性削弱大陆风化通量的同时,使海洋化学更强烈地受制于海底热液交换与封闭水体过程(Dutkiewicz et al., 2024;Thomas and Catling, 2024)。冰期结束后,构造运动由离散转向汇聚,泛非造山作用广泛发育,标志着冈瓦纳大陆逐步形成,大陆风化通量及其化学效应显著增强(Bowyer et al., 2023)。这一完整的“裂解—冰期—再聚合”的构造-气候循环,为新元古代海水化学与氧化还原格局的阶段性转型提供了根本背景(Li et al., 2020)。

在上述地球系统剧烈重组的背景下,新元古代同时成为地质记录中白云石异常富集的重要时期(Hood et al., 2011;Crockford et al., 2021)。该时期广泛发育原生或准同生白云石,其沉积-成岩时序往往与全球构造-气候事件密切对应(Zhou et al., 2025)。然而,这些白云石的形成与现代海水条件下白云石难以沉淀的实验和观测结果之间存在显著矛盾,使得“白云岩问题”在新元古代集中体现(Burns et al., 2000;王金艺和金振奎,2022)。尽管已有研究从无机化学、微生物诱导及孔隙水演化等角度对该问题进行了探索,但在关键控制因素及其全球适用性方面仍存在明显分歧(García-Ruiz,2023;Wang et al., 2023)。人工合成实验与天然实例表明,硫酸盐还原菌等微生物可通过改变局部碱度、降低Mg²⁺水化能垒并促进晶体成核,从而提高白云石沉淀概率(Jiang et al., 2024;Liu et al., 2024)。部分学者认为,前寒武纪海洋可能具有更高的Mg/Ca值及不同的离子结构,从热力学上扩大了白云石稳定区间,使其更易形成(Chen et al., 2023)。另一类研究强调蒸发浓缩、混合水或孔隙流体循环可在沉积后阶段触发白云石化过程(Adineh et al., 2025)。然而,不同盆地中白云石的形成时序差异明显,单一成岩路径难以解释其普遍性(Bowyer et al., 2025;Liu et al., 2025)。总体来看,当前争论的核心并非“是否存在某一机制”,而在于哪些因素属于全球尺度的背景条件,哪些是直接触发沉淀的局地过程。

与此同时,不同地区新元古代白云石在晶体形态、地球化学特征及沉积背景上的差异,也表明其形成过程并非单一机制所能够解释(Wang et al., 2020)。在此背景下,Hood等(2011)提出了区别于显生宙“文石—方解石海”体系的“白云石海”模式,即在高Mg/Ca值、低硫酸盐、高碱度及弱氧化环境共同作用下,新元古代海洋可能处于一种系统性促进白云石沉淀的特殊状态(Hood et al., 2011;Wang et al., 2020;Liu et al., 2025)。该模型为新元古代白云石富集提供了重要解释框架,但其是否代表一种长期稳定的海洋状态,抑或仅在特定构造-气候阶段间歇性出现,仍是当前亟待约束的关键科学问题。

因此,系统厘定新元古代海水化学条件及其对白云石沉淀的控制机制,对于深化对白云石成因问题和古海洋化学演化过程的认识具有重要意义。在综合全球不同地区新元古代地层记录的基础上,笔者系统梳理了岩石学与多同位素地球化学等多学科证据,重点评述海水化学体系及氧化还原结构的阶段性演化特征,并探讨其与构造活动、冰期过程及大陆风化通量变化之间的耦合关系。从“背景条件—触发过程”的综合视角出发,提出新元古代海水化学由“热液主控”向“风化主控”转型的阶段性演化框架,旨在从海水化学条件约束的角度,解释白云石异常富集的形成背景,并为理解新元古代地球表层系统的整体重组提供参考。

1 新元古代地层及其白云石沉淀背景

原生海相白云石胶结物并非零散或局地发育,而是在多个相互独立的沉积体系中反复出现,其出现层位与沉积背景具有明确的可对比性(Lloyd et al., 2020;Mtonda et al., 2024)。对全球新元古代海相白云石胶结物的时代—地层—沉积微相进行统计发现,其发育环境涵盖浅水台地、潮坪—蒸发台地、台缘—斜坡以及深水盆地等多种沉积体系,而并未明显集中于局限台地环境(表1)。除部分潮坪与蒸发台地外,大量白云石胶结物发育于开阔海缓坡、台地边缘带及深水盆地沉积背景中,显示其形成并不受单一水深或特定沉积相带严格控制。这一全球分布格局在多个典型地区得到一致体现。例如,在中国南方扬子地台、澳大利亚Adelaide褶皱带、纳米比亚Otavi褶皱带以及美国死亡谷等地区,均可在碳酸盐岩内部识别出海相白云石胶结物(图2)(Hood et al., 2011;Shuster et al., 2018;Wood et al.,2018;Zhou et al., 2025)。这些胶结物既发育于潮坪和浅水台地环境,也可见于台缘礁体空腔及斜坡—盆地过渡带,进一步表明其沉积背景具有跨相带特征(Hood and Wallace,2018;Zhang et al., 2022b)。与显生宙大规模白云岩多集中于高蒸发、局限台地环境的沉积模式不同,新元古代原生海相白云石胶结物的全球分布更指示其形成可能主要受当时广泛存在的海水化学条件控制,而非单一沉积微相决定(Hood et al., 2011;Spence et al., 2016)。

从岩石学特征来看,不同地区白云石胶结物在晶体形态和生长方式上表现出高度一致的特征(图3)。无论是澳大利亚Nuccaleena组发育的菱形宽叶片状白云石胶结物、纳米比亚Rasthof组中的径向白云石胶结物、美国Beck Spring 组束状和放射状白云石胶结物的叠加,还是扬子地台灯影组中发育的葡萄状或束状胶结壳,其共同特征均表现为白云石沿孔隙壁等厚连续生长、晶体细长且定向一致(Shuster et al., 2018;Liu et al., 2025)。这类结构明显不同于后期成岩阶段形成的粗晶或交代型白云石,而更符合同沉积至极早期成岩条件下直接沉淀的特征,指示其形成过程与同期海水化学条件密切耦合(Wood et al., 2018)。

在综合绝对年龄约束、关键标志层及岩性组合的基础上,碳同位素记录进一步强化了4个地区地层之间的全球一致性(图2)。在这些新元古代剖面中,δ¹³C曲线在多个关键层位表现出可对比的变化趋势,尤其是在Sturtian与Marinoan冰期结束后的盖帽状碳酸盐层中,普遍出现幅度相近的负δ¹³C异常,并与绝对年龄及冰期标志层具有良好对应关系(Busch et al., 2022;Mitchell et al., 2023)。此外,在部分埃迪卡拉纪地层中可识别出区域至全球尺度的负偏事件,进一步表明新元古代海洋碳循环在全球范围内经历了多阶段显著扰动。这类跨大陆同步出现的碳同位素事件更可能反映了新元古代海水溶解无机碳库在全球尺度上发生来源、规模或同位素组成的系统性变化(Mitchell et al., 2023)。

不同地区白云石胶结物形态组合表现出一定的区域差异,主要体现在晶体聚集方式和空间组合形式上(Zhao et al., 2021;Lu et al., 2023)。例如,Nuccaleena组中宽阔菱形放射状晶体及明显世代分带反映了较为开放的孔隙体系和持续的流体补给;Rastho组中径向纤维状白云石常见于相对封闭的孔隙或空腔环境,显示快速结晶特征(Wallace et al., 2015);Beck Spring组中多世代纤维—放射状白云石叠置关系则记录了孔隙流体化学条件的阶段性变化及微生物微晶基底对成核过程的潜在影响(Shuster et al., 2018);灯影组中白云石胶结物常与微生物构造共生,指示成核过程可能受到微生物基质的影响(李安鹏等,2023)。这些差异主要反映局部沉积动力条件与孔隙环境的调节作用,并不改变其总体属于海相早期胶结物的本质属性。

综上,不同大陆新元古代地层中原生海相白云石胶结物的反复出现,结合其稳定的岩石学特征及可对比的碳同位素背景,支持其形成受控于当时普遍存在的海水化学条件,而非孤立的局地成岩事件。这一跨大陆的地层与矿物学一致性,为在全球尺度上利用白云石胶结物重建新元古代海水化学状态提供了坚实的地层学基础。

2 海相白云石胶结物岩石学特征及其成岩指示意义

新元古代沉积序列中广泛发育的海相白云石胶结物,被普遍认为是重建古海水化学条件的重要矿物记录(Hu et al., 2020)。大量研究表明,该时期白云石化过程多发生于沉积至早期成岩阶段,因而受到后期深埋成岩和流体交代的系统性改造相对较少,能够在一定程度上保留沉淀时孔隙水—海水体系的化学信息(Crockford et al., 2021;Tang et al., 2023;闫斌等,2024)。在岩石学上,新元古代白云岩通常由原始沉积形成的白云石基质与沉积后形成的白云石胶结物共同组成。

基质白云石(Matrix dolomite,MD)在新元古代碳酸盐沉积体系中分布广泛,是构成碳酸盐岩基质的重要矿物组分(蒋裕强等,2016;谷一凡等,2024)。该类白云石通常形成于沉积期至同生—准同生阶段,常与微生物沉积构造密切伴生。在宏观尺度上,基质白云石可表现为叠层构造、凝块构造等典型微生物诱导沉积构造(图4-A,4-B,4-C),镜下可识别叠层结构、凝块结构、泡沫绵层结构、核形石及鲕粒等特征(图4-D至4-I),反映出其形成环境中活跃的微生物作用背景。基质白云石主要记录了浅海碳酸盐体系中早期沉积环境的整体特征(Song et al.,2024)。尽管基质白云石在成岩演化过程中可能经历不同程度的再结晶或交代改造,但在微生物构造形态与岩相组合得以保留的前提下,其仍可为新元古代碳酸盐沉积体系的环境背景与早期成岩条件提供重要约束。

白云石胶结物是指在孔隙、裂隙或组构间隙中由孔隙流体沉淀形成的白云石,其形成时序通常晚于原始基质沉积,但总体仍处于同沉积—早期成岩阶段。该类白云石更直接地反映了孔隙水—海水体系在沉淀过程中的化学过饱和状态及流体演化特征(鲁鹏达等,2023)。在新元古代海相碳酸盐岩中,白云石胶结物可表现为等轴状、镶嵌状或纤维状等多种形态,其中纤维状白云石胶结物(Fibrous dolomite,FD)最具成因和环境指示意义。纤维状白云石胶结物通常沿孔隙、裂隙或生物骨架内壁呈等厚环带或结壳状生长,在剖面上显示连续的纹层或环边壳状结构,在横切面上则常呈现葡萄状、串珠状或放射状几何形态。

根据晶体光学延性及生长结构特征,纤维状白云石胶结物可进一步划分为原生型与成岩改造型2类(Shuster et al., 2018;Zhao et al., 2021;甯濛等,2025)。其中,原生型主要包括束状正延性白云石胶结物(Fascicular slow dolomite,FSD)和放射状正延性白云石胶结物(Radial slow dolomite,RSD)(图5)。该类白云石胶结物常作为第1代孔隙胶结物出现,并与早期沉积构造保持同轴连续生长关系。其晶体形态规则,纤维或放射状结构连续,未见明显交代边界(图5-A至5-F)。在阴极发光下,FSD与RSD普遍发育清晰的节律状发光带,表现为平行于基底的明暗条带交替分布,且晶体末端通常缺乏清晰的菱面体终止面,反映出沉淀过程的连续性与同生性。这些特征被广泛视为判识原生海相白云石胶结物的重要依据,表明其形成于同沉积至极早期成岩阶段,较少受到后期成岩作用改造。

相比之下,成岩改造型FFD与RFD常表现为局部充填、晶体形态不稳定,并普遍伴随重结晶、过生长或交代等成岩特征(图5-G至5-J),其形成受后期孔隙流体演化和成岩条件变化的控制较强,地球化学组成难以代表原始海水信号(Shuster et al., 2018;Zhao et al., 2021)。在重建新元古代海水化学条件时,已有研究普遍强调应优先选取具有明确原生特征的FSD与RSD,并谨慎排除后期成岩改造型白云石,以提高不同地区研究结果的可比性和解释的可靠性。本文后续关于新元古代海水化学演化的讨论,也将优先依据具有明确原生特征的白云石胶结物所记录的信息展开。

3 多同位素约束下的新元古代海水化学条件

3.1 碳氧同位素对碳循环扰动与沉积环境变化的指示

碳氧同位素可用于重建新元古代海水化学演化与全球碳循环变化,在约束海洋碳库规模、碳通量重组及沉积—成岩过程方面具有关键作用(Halverson et al., 2005;Zou et al., 2024)。在本文中,这2类同位素并非用于单独讨论碳循环演化过程本身,而是作为判别白云石胶结物形成背景的重要约束条件,用以区分全球尺度碳库扰动与局地沉积—成岩效应在白云石沉淀过程中的相对作用。

从全球尺度看,非冰期海相白云石胶结物普遍表现为δ¹³C偏正(约0~8‰)(图6)。这一特征在多个相互独立的台地体系中反复出现,反映了有机碳长期净埋藏增强背景下,海洋溶解无机碳库向重同位素富集的结果,碳循环处于相对稳定但通量较高的状态(Johnston et al., 2012;Fernandes et al., 2024)。该类δ¹³C特征的跨区域一致性表明,新元古代海水环境在全球尺度上受到统一碳库背景的控制,为白云石沉淀提供了可对比的地球化学框架。δ¹⁸O值表现为中等偏负(约-10‰~0),且在不同地区之间存在一定离散(图6)。δ¹⁸O相较于δ¹³C更易受到沉积环境开放性、水体封闭程度及早期成岩过程的调控(严兆彬等,2005)。然而,其整体分布范围仍与新元古代海水组成保持一致,说明在沉积期—早期成岩阶段,白云石胶结物总体仍处于海水控制体系之内(Zou et al., 2024;Wang et al., 2025)。

冰期结束后的盖帽白云石显示出显著负偏的δ¹³C值(约-6‰~0)(图6)。该负异常在全球多套地层中同步出现,通常指示冰期后阶段海洋溶解无机碳库组成在短时间尺度内发生显著变化(Corsetti and Kaufman,2005;Wu et al., 2019)。盖帽白云石在此阶段的大规模发育,表明白云石沉淀对全球碳循环扰动具有高度敏感性(Busch et al., 2022)。然而,盖帽白云石的 δ¹⁸O 值表现出明显的区域分异,分布于2个区间(约-18‰~-12‰与-10‰~-2‰)(图6)。其中,斜坡—外陆棚环境中极轻的 δ¹⁸O 值更可能受到沉积速率快、孔隙水封闭及早期成岩缓冲作用的影响,难以代表原始海水组成(Nelson et al., 2021)。而浅水台地环境中形成的白云石,由于与海水保持较开放的交换条件,更可能记录冰期后快速回暖与蒸发增强背景下的水体特征(Busch et al., 2022;Tang et al., 2023)。这种δ¹³C全球一致、δ¹⁸O区域分异的组合关系表明,全球海水条件是控制新元古代同位素演化的一级因素,而局地沉积环境则对 δ¹⁸O 起到二级调控作用。

从时间演化角度看,新元古代碳同位素记录呈现出一系列幅度显著的全球性扰动事件(图7),如Majiatun异常、Bitter Springs异常、Islay异常及Shuram异常等,反映了全球碳库在不同构造-气候阶段的多次重组(Mitchell et al., 2023;Tasistro-Hart et al., 2025)。这些事件与白云石胶结物在地层中的集中出现具有良好的对应关系,进一步表明白云石沉淀并非随机发生,而是倾向于在全球碳循环发生显著扰动的阶段被触发。

3.2 ⁸⁷Sr/⁸⁶Sr值与海水端元输入演变

锶同位素组成能够有效约束海水物质输入结构及其长期演化(Weldeghebriel et al., 2023;Mtonda et al., 2024)。锶在海水中的停留时间(约2~3 Ma)显著长于海洋混合时间(约1 kyr),因此海水⁸⁷Sr/⁸⁶Sr值能够有效记录全球尺度上大陆风化输入与海底热液交换之间的相对平衡(Halverson et al., 2007)。

全球编录数据显示(图7),新元古代海水⁸⁷Sr/⁸⁶Sr值整体呈持续升高趋势,由早期的0.7055逐步升至埃迪卡拉纪的0.7085以上(Hessler et al., 2017)。这一长期演化轨迹反映了地球表层系统由热液交换主控向大陆风化输入主控的根本性转变,与罗迪尼亚超大陆的裂解、冰期—间冰期气候振荡以及冈瓦纳大陆聚合过程密切相关,表明构造重组通过调控物质输入方式,对海水化学结构产生了持续而深远的影响(Chen et al., 2022;Mtonda et al., 2024)。

在新元古代早期,海水⁸⁷Sr/⁸⁶Sr值维持在相对较低且稳定的水平,锶组成主要受洋中脊热液交换和低放射性基性—超基性岩风化控制(Chen et al., 2022)。这一阶段对应罗迪尼亚超大陆相对完整但内部裂谷广泛发育的构造背景,新生洋壳形成速率较高,热液通量在全球物质循环中占据重要地位(Zou et al., 2024)。进入拉伸纪晚期至成冰纪,伴随着超大陆逐步裂解和2次全球冰期的发生与消融,大陆风化通量在时间上呈现强烈波动(Tasistro-Hart et al., 2025;Yang et al., 2025)。海水⁸⁷Sr/⁸⁶Sr值记录到2个明显的上升拐点,对应2次冰期消融阶段,反映冰盖退缩后大陆快速暴露和风化输入增强,使放射成因锶短时间内大量进入海洋。至埃迪卡拉纪早期,海水⁸⁷Sr/⁸⁶Sr值达到新元古代最高值,指示大陆风化已成为主导性锶输入端元,对应冈瓦纳大陆聚合及泛非造山作用背景下的长期构造抬升与剥蚀增强阶段(Zhou et al., 2020;Zhang et al., 2022c)。

Sr同位素将新元古代白云石沉淀发生的构造背景,限定在“热液主控—裂解增强—大陆聚合”这一全球尺度的构造演化序列之中。这一约束表明,新元古代白云石的广泛发育并非发生在单一、静态的构造背景下,而是在不同构造阶段对应的不同物质输入体系中多次出现。

3.3 Mo-U同位素约束的海洋氧化还原结构

钼(Mo)、铀(U)同位素因在海水中具有较长的停留时间,并在不同氧化还原条件下表现出系统性的分馏行为,被广泛用于约束古海洋氧化还原状态及其空间分异特征(Zhang et al., 2022a)。Mo、U 同位素对氧化还原环境的响应机制存在显著差异,其环境指示意义必须建立在明确的沉积前提与质量平衡框架之上(Wei et al., 2021;O’Sullivan et al., 2022)。

对于Mo同位素而言,其作为广海氧化还原状态示踪指标的前提是沉积环境处于硫化缺氧条件(Lu et al., 2020;Nan et al., 2023)。在该环境中,海水中溶解态钼以硫代钼酸盐形式被定量去除并埋藏,沉积物δ⁹⁸Mo值可较好地继承当时海水端元组成,从而反映全球海洋中硫化水体的空间占比(Kendall et al., 2015;Wei et al., 2021)。若沉积环境仅为非硫化缺氧或弱氧化条件,Mo的去除过程往往不完全,其同位素分馏易受局地吸附、铁锰氧化物循环及孔隙水过程影响,此时沉积物δ⁹⁸Mo难以代表广海氧化还原状态(Nan et al., 2023)。与Mo不同,U同位素对缺氧环境的响应主要体现在全球质量平衡层面(Li et al., 2024b)。在氧化海洋中,U以U(Ⅵ)形式溶解存在,而在缺氧、尤其是硫化环境中,U(Ⅵ)发生还原并被高效埋藏,优先富集轻同位素,从而驱动海水δ²³⁸U向轻值偏移(Tissot and Dauphas,2015)。因此,海水及其近源沉积物中的δ²³⁸U变化,对全球范围内缺氧(尤其是硫化)海域面积的变化高度敏感,可用于定量约束海洋整体氧化还原状态的演化趋势(Lu et al., 2020;Wei et al., 2021)。

在拉伸纪至成冰纪早期,海水Mo与U同位素整体偏轻,指示当时海洋体系广泛发育缺氧环境(图7)(Huang et al., 2023)。该特征与超大陆裂解背景下营养盐输入增强、初级生产力提高及水体分层加剧的认识相一致(Tostevin and Mills,2020;Sun et al., 2025)。随着冰期结束,Mo-U同位素记录呈现出显著的空间分异特征,海洋氧化还原结构由相对均一的缺氧状态向明显分层格局转变(Kendall et al., 2015;Wei et al., 2021)。一方面,部分浅水碳酸盐岩(如Shuram事件相关层位)中δ²³⁸U接近现代海水值(-0.39‰±0.005‰),表明浅表海洋经历了阶段性的氧化程度提升(Tissot and Dauphas,2015;Andersen et al., 2016)。另一方面,深水或半封闭沉积体系中仍普遍发育极轻的δ⁹⁸Mo和δ²³⁸U,揭示深层水体在相当长时间内仍维持缺氧状态(Zhang et al., 2024a)。这种浅层与深层之间的同位素分异,支持新元古代晚期海洋具有“表层逐步氧化、深层持续还原”的分层氧化还原结构(Clarkson et al., 2023;Li et al., 2024b)。

3.4 Mg同位素与Mg/Ca值揭示的富镁海水背景

Mg同位素记录了镁在不同端元的输入差异及其在碳酸盐沉淀过程中的分馏效应,Mg/Ca值主要反映海水中Mg与Ca的相对供给与去向平衡(陈洁等,2021;Xia et al., 2024)。因此,两者的协同变化为识别新元古代海水化学背景及白云石形成条件提供了关键约束(Ries,2010;Guo et al., 2019;王芮等,2026)。

从长期演化趋势来看,新元古代海水Mg/Ca值整体呈持续升高特征。这一趋势通常被认为与超大陆裂解背景下大陆边缘暴露面积扩大、陆源硅酸盐风化增强以及洋中脊热液交换相对减弱等过程有关(Higgins and Schrag,2015)。Mg/Ca值的升高改变了海水中碳酸盐矿物的稳定场,使文石与高镁方解石更易沉淀,并在特定化学条件下为低温白云石的形成提供必要的离子基础(Ries,2010;Zhang et al., 2022b)。尽管冰期阶段可能因短期Ca²⁺通量增强而对Mg/Ca值产生扰动,但在冰期消融后陆源风化通量快速恢复的背景下,高Mg/Ca值状态仍能作为新元古代海洋的长期化学特征得以维持(Kasemann et al., 2014;Ahm et al., 2021)。整体而言,这一时期的海水呈现出相对于早期元古代和显生宙初期更为突出的富镁背景(Wang et al., 2020)。

Mg同位素记录为上述海水化学演化提供了过程层面的补充约束(图7)(Guo et al., 2019)。现有研究表明,新元古代海水δ²⁶Mg值整体偏轻,其变化可能反映了不同镁输入端元的相对贡献差异,包括海底热液供镁、陆源风化输入以及局限海盆内的水体交换等因素(Higgins and Schrag,2015)。进入成冰纪后,随着大陆风化作用逐步增强,陆源镁输入在海洋镁循环中的权重提高,海水δ²⁶Mg值呈现出阶段性升高趋势,指示海水镁循环逐渐由热液端元主导向陆源端元贡献增强的方向演化(Kasemann et al., 2014;Ahm et al., 2021)。

在碳酸盐沉淀过程中,白云石对Mg的同位素分馏效应使其δ²⁶Mg相对于海水产生系统性偏移,这一特征在全球新元古代白云岩中具有较好的可识别性(Tipper,2022;Lin et al., 2023)。尽管单个白云石样品的Mg同位素组成难以直接代表同期海水端元,但其在区域尺度上的一致变化,仍可反映当时海洋中镁沉淀过程的活跃程度以及白云石形成在海水镁循环中的重要地位(Merdith et al., 2017;Li et al., 2024c)。特别是在高Mg/Ca值背景下,白云石沉淀对镁的持续消耗,可能进一步放大海水化学条件与白云石形成之间的耦合效应(Zhang et al., 2022b)。

综合Mg/Ca值与Mg同位素证据,新元古代海洋经历了由相对低 Mg/Ca值向持续富镁状态的转变。在此化学环境下,海水碳酸盐体系更容易进入有利于低温白云石形成的稳定窗口期(陈洁等,2021;Lin et al., 2023)。该富镁海洋背景不仅对白云石胶结物和原生白云岩的广泛发育具有控制作用,也为埃迪卡拉纪海洋化学趋稳及其后的生物演化奠定了重要的环境条件(Hood et al.,2011;Hood and Wallace,2018)。

4 新元古代海水化学演化及其对白云石沉淀的约束

4.1 构造-气候驱动下的海水化学与氧化格局

多种同位素地球化学记录与全球地层对比表明,新元古代海洋并非沿单一方向连续演化,而是在构造重组与极端气候事件共同驱动下呈现出明显的阶段差异。因此,从控制海水化学的主要物质通量出发,分别讨论海底热液交换占优、冰期封闭扰动以及大陆风化增强3个关键背景下,海水离子组成、硫酸盐水平与氧化还原结构的变化特征,以厘清新元古代海洋化学演化的基本脉络。

在拉伸纪至成冰纪早期(图8-A),罗迪尼亚超大陆的持续裂解显著增强了洋中脊扩张与海底热液活动,海水—洋壳之间的高频交换是控制海水化学结构的主要过程(Li et al., 2008)。高温热液过程持续移除Ca,而低温洋壳蚀变与热液循环维持较高的Mg通量,使海水Mg/Ca值处于整体偏高状态(Xia et al., 2024)。尽管同期大陆裂谷发育和局部构造抬升促进了化学风化,但风化对象以年轻的基性—中性岩石为主,其Sr与Mg同位素特征难以显著改变全球海水端元组成(Tipper,2022)。Mo-U同位素普遍偏轻,指示深水环境长期处于缺氧状态,表明新元古代早期海洋尚未建立稳定的分层氧化结构,整体呈现出“热液主控型”海洋化学格局(Tostevin and Mills,2020;O’Sullivan et al., 2022)。

随后,2次雪球地球冰期及其间冰期构成了新元古代海水化学演化中最为剧烈的振荡阶段(图8-B)(Li et al., 2023;Thomas and Catling,2024)。冰期期间,广泛的冰盖覆盖使大陆风化与河流输入几近停滞,海洋体系趋于高度封闭(Dutkiewicz et al., 2024)。海水化学主要受热液端元与内部再循环控制,硫酸盐浓度进一步降低,Mg/Ca值在Ca输入受限的条件下维持在相对较高水平,Mo-U同位素极轻值反映深海氧化层的显著萎缩(Ahm et al., 2021)。冰期消融后的短暂回暖阶段则伴随着大陆快速暴露和风化通量的突发增强,大量放射性锶、溶解无机碳及营养盐被输送至海洋,从而导致海水碱度显著升高,并引发碳循环、硫循环及海水化学结构的快速重组(Kasemann et al., 2014)。⁸⁷Sr/⁸⁶Sr值的陡升、显著的δ¹³C负异常以及Mo-U同位素的区域分异,共同记录了这一非稳态振荡过程(Yang et al., 2020;Chen et al., 2022)。

进入埃迪卡拉纪中晚期(图8-C),随着冈瓦纳大陆逐步聚合和泛非造山作用的广泛发育,大陆风化通量在强度及其地球化学效应上均显著增强(Merdith et al., 2017)。持续升高的海水⁸⁷Sr/⁸⁶Sr值及相对稳定偏重的δ²⁶Mg值,表明大陆风化已取代海底热液交换,成为控制海水化学组成的主导端元(Li et al., 2013;Peng et al., 2025)。与此同时,Mo-U同位素记录显示海洋逐步建立起稳定的分层氧化结构: 表层水体趋于氧化,而深层水体仍维持缺氧状态(Zhang et al., 2018)。这一阶段的海水化学演化标志着新元古代海洋由“热液主控”向“风化主控”的根本性转变。

4.2 海水化学条件对白云石沉淀的阶段性制约

在上述海水化学演化背景下,新元古代白云石的广泛发育并不指示一个长期稳定存在的“白云石海”,而是反映了白云石沉淀对海水化学条件阶段性满足的响应。白云石沉淀受热力学条件与动力学过程的协同制约,需要富镁背景、相对受限的硫酸盐水平、充足的碱度供给以及适宜的氧化还原条件在特定时间和环境中同时具备(Chen et al., 2023;García-Ruiz,2023;Meister et al.,2023)。此外,温度通过调节Mg²⁺脱水与晶体生长速率,对白云石沉淀的动力学具有基础性影响,但其作用通常需要在富镁、高碱度及低硫酸盐等化学条件共同满足时方能有效体现(Arvidson and Mackenzie,1999)。当这些约束被满足时,才形成短暂而有效的白云石沉淀窗口。

新元古代白云石的形成虽在一定程度上受沉积微相影响,但其控制作用明显弱于显生宙蒸发台地模式(Tang et al., 2023)。新元古代海水整体更易达到对白云石过饱和状态,使白云石沉淀不再严格局限于潮坪或潟湖等强蒸发环境,而可广泛出现在开放台地、台地边缘甚至斜坡等多种沉积体系中(Crockford et al., 2021)。因此,新元古代白云石的空间分布更多反映全球海水化学条件的阶段性有利,而非单一沉积微相的局部控制。

在以热液交换为主导的海水体系中(图8-A),整体富镁、低硫酸盐且碱度较高的背景条件为白云石成核提供了长期存在的化学基础(Adineh et al., 2025)。在浅海台地及微生物活动较为活跃的环境中,这种背景有利于细晶—微晶白云石的缓慢形成,常呈基质型或早期孔隙填隙状(Lu et al., 2023;Zhuang et al., 2025)。该类型白云石沉淀过程相对连续,但成岩速率有限,表现为背景性、广泛分布的沉积特征。

在极端气候扰动背景下(图8-B),白云石沉淀则表现出明显的事件性特征。冰期形成的高Mg/Ca值、低硫酸盐背景在冰期后阶段并未立即被完全重置,而是与突发增强的大陆风化输入叠加,使浅海环境在短时间内达到高度过饱和状态(Fang and Xu,2022;Thomas and Catling,2024)。与此同时,由冰室向温室的快速转变所引发的海水升温,可能从动力学层面降低Mg²⁺脱水能垒并加快晶体成核速率,从而显著提升白云石沉淀的可行性。实验与理论研究均表明,温度升高有助于突破低温条件下白云石形成所面临的动力学障碍,使原本仅具化学过饱和条件的海水体系更容易进入实际沉淀窗口(Chen et al., 2023)。此时,大量白云石在短时间内快速沉淀,并常与显著的δ¹³C负异常及Sr同位素突变共生(Zhang et al., 2024b)。因此,冰后快速回暖不仅重塑了海水离子结构,也可能通过增强矿物沉淀动力学,对新元古代白云石的事件性富集产生放大作用。

随着大陆风化作用持续增强和海洋分层氧化结构的逐步建立(图8-C),白云石沉淀由早期的广泛背景过程转变为受环境严格限制的局部现象(Zhang et al.,2021)。此阶段,即便海水仍保持相对富镁特征,硫酸盐水平和氧化条件的变化已对白云石形成产生更强约束,使其主要局限于浅海台地、半封闭水体或微生物作用显著的沉积环境中(Huang et al., 2024;Li et al., 2025)。白云石多以纤维状或放射状胶结物形式出现,表明白云石沉淀过程高度受控于局地化学条件与沉积环境(林孝先等,2015;Zhao et al., 2021)。

总体而言,新元古代白云石的异常富集并非源于单一化学状态的长期维持,而是海水化学体系在构造重组、气候扰动和物质通量变化背景下,多次短暂进入有利于白云石沉淀的状态。白云石记录的核心信息,在于这些“沉淀窗口”被反复打开的过程本身,而非某一参数的绝对数值。

5 结论与展望

多同位素证据表明,新元古代海水化学并非处于静态平衡,而是在构造重组与冰期过程的共同作用下持续演化,整体趋势表现为由早期受洋中脊热液交换显著影响过渡至冰期—消融阶段的强烈化学波动,并在晚期逐步转向以大陆风化输入为主导的海水化学背景。这种长期演化并未从根本上改变白云石形成所需的基本化学前提,而是通过调节硫酸盐水平及海洋氧化还原结构,阶段性地开启白云石沉淀窗口,从而限制其发生的空间范围和环境条件。新元古代白云石的异常富集并非连续过程的产物,而是发生于特定海水化学条件组合被短暂开启的“沉淀窗口期”。当这些窗口在特定沉积环境(如浅水台地或受限海域)中被反复触发时,即可形成具有代表性的原生白云石胶结物组合。因此,新元古代白云石的广泛发育反映的并非单一成因机制的增强,而是在总体富镁海水背景下,白云石沉淀对白云石成核环境、孔隙流体化学及氧化还原条件的高度敏感综合响应。

尽管基于多同位素体系对新元古代海水化学演化及白云石异常富集机制进行了综合讨论,但仍存在若干有待深入的问题。未来研究可从以下几个方面推进:

1)在高精度年代学框架约束下,加强同一剖面、同一沉积单元内多同位素指标的协同分析,以提高不同地区记录在时间尺度上的可比性,进一步削弱区域沉积环境和成岩作用对全球海水信号判读的干扰。

2)深化对白云石沉淀过程中无机化学条件与微生物作用关系的认识,结合原位同位素测试、团簇同位素及非传统稳定同位素等手段,从微尺度约束白云石成核环境、流体来源及沉淀速率,明确微生物活动在早期白云石形成中的调节作用。

3)在现有阶段性海水化学演化认识的基础上,逐步构建海水化学参数的半定量或定量重建方法,结合高分辨率地层对比与数值模拟,尝试建立海水化学变化与白云石发育层位之间的时间对应关系,从而进一步检验海水化学波动对白云石沉淀的控制强度及其时空耦合机制。

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基金资助

*国家油气重大专项(2025ZD1406403)

中国海洋石油集团有限公司中国—巴西科技创新中心项目(KJZH-2025-2406)

国家自然科学基金项目(42372146)

国家自然科学基金项目(41972107)